1. Field of the Invention
This invention relates to an f-.theta. lens, and more particularly, to an f-.theta. lens suitable for use in a diode laser printer which includes a rotating polygon scanner.
2. Description of the Prior Art
Laser printers are used in photography and in the graphic arts for printing on a receiving medium such as film. When such printers are used for color imaging, they generally include a separate light source for each of the three primary colors. Light beams from the separate sources are combined into a colinear combination beam, and this beam is then scanned over an angular range by a rotating polygon. An f-.theta. lens is located in the optical path between the polygon and the receiving medium.
The input beam to the polygon and to the f-.theta. lens would normally be collimated in all three colors or wavelengths. The f-.theta. lens would also normally be designed to correct both axial and lateral color at the image plane, assuming a collimated input beam.
Such an f-.theta. lens, however, may not be suitable for use with a polygon scanner which has pyramidal angle error on one or more of its individual facets. A facet having this type of error is tilted so that its face, if extended, would intercept the axis of rotation of the polygon. If there is no pyramidal angle error, the plane of the facet is parallel to the axis of revolution. In the case of zero pyramidal angle error, an input beam which lies in a plane perpendicular to the axis of rotation will, after reflection off the facet, remain in this plane. In the case of non-zero pyramidal angle error, the beam after reflection will no longer lie in this plane, and after passing through the f-.theta. lens, the beam will be focused at the image plane at a location either above or below the scan line; this is referred to as cross-scan error. If the input beam consists of three different colors or wavelengths, the amount of cross-scan error may be different for each wavelength. These wavelength-dependent cross-scan errors could cause a problem known as banding.
In U.S. Pat. No. 4,396,254, for example, there is disclosed an f-.theta. lens which includes means for color correction, over a wavelength region of 632.8 to 1064 nanometers. However, there is no suggestion in this patent that the disclosed lens is corrected for pyramidal angle error on a polygon facet, and thus, the lens would not be suitable for use in a laser color printer which uses a rotating polygon to scan the beam.